1. Field of the Invention
The present invention relates to mounting arrangements for high voltage/high power semiconductors, and in particular for GTO thyristors with bypass diodes.
2. Description of the Related Art
Traction vehicles such as rail transit vehicles are frequently powered by either direct current (DC) or alternating current (AC) electric motors. The power developed by such motors may be as high as 1000 horsepower (HP) per motor. Such high HP motors require commensurately high, controlled electric power. For example, a nominal power requirement may be 750 volts at 1000 amperes when the motor is operated in a propulsion mode. The motors also typically are used for electrical braking of the traction vehicle by operating them as generators, which may produce even higher voltages and currents. In the case of an AC electric motor operating as a generator, the peak voltage routinely exceeds 1000 volts.
Power control systems for such electric traction motors typically use power semiconductor devices, such as gate turn off (GTO) thyristors, to control power flow to and from the motors. Due to the magnitude of the power being controlled, there is a significant amount of heat that must be dissipated by the semiconductor devices. This heat is generally dissipated by mounting the semiconductors on relatively large heat sinks. The heat sinks are metal conductors having good thermal characteristics and sufficient surface area to dissipate sufficient heat to maintain the semiconductors within their operating temperature limits.
The semiconductors are connected to high potentials. For many applications, it is desirable for safety reasons that such large heat sinks be maintained at electrical ground potential. Thus, some form of electrical insulation may be provided between the semiconductors and the heat sinks. However, any such electrical insulation must be a good thermal conductor, since it is positioned between the semiconductors and the heat sink. The insulation conventionally used is a sheet material available under the trade name of Chotherm, which is believed to be a boron nitride material with a silicon binder. In a conventional application, each semiconductor is clamped to the heat sink, with the Chotherm sheet clamped therebetween.
The AC traction drive power supply commonly used in transit vehicles is a voltage source pulse width modulated GTO thyristor inverter. The switching is done by the GTO thyristor, but a bypass diode is placed in antiparallel across the GTO thyristor due to the inductive nature of the traction motor load. A snubber diode and snubber capacitor also are normally needed to limit voltage during turn off. Finally, a remotely located resistor normally is connected to dissipate the energy trapped in the capacitor.
Semiconductors of this power requirement are preferably packaged in "press packs" in which the cathode and anode terminals are large flat surfaces on opposite ends of the semiconductors. The GTO thyristor and diode commonly are mounted to the heat sink side by side using separate clamps. This side by side mounting of the already large press packs requires a large heat sink and a large area of insulating material.